This invention relates to a receiving apparatus for a remote control system, more particularly to a receiving system for a remote control system especially suitable for various alarm systems utilizing a bidirectional transmission line of a wired television broadcasting system. (hereinafter termed CATV system)
The applicant has already proposed an alarm system utilizing the CATV system as shown in U.S. Pat. No. 3,840,812. In this system the alarm signals from respective subscriber terminals are sent as low frequency signals by alarm signal transmitters, the low frequency signals are grouped for respective floors of a building or respective street blocks for modulating carrier wave having high frequencies specific to respective floors or street blocks to form up-signals (hereinafter termed group signals) and each group signal is transmitted to a remotely located alarm receiving apparatus as frequency divided multiple signals (hereinafter termed super group signals).
The alarm receiving apparatus is provided with a circuit for discriminating a particular subscriber terminal among a group of subscriber terminals from which the alarm signal has been transmitted, said circuit comprising a group demodulator including a bandpass filter for high frequency carrier waves and a filter for low frequency signals. The group demodulator including the high frequency bandpass filter operates to demodulate the super group signals which have been produced by modulating respective carrier waves by a group of low frequency signals of each generated in respective blocks or floors. Each group demodulator corresponds to each group modulator and the numbers of the group modulators and group demodulators are equal. Each group signal demodulated by the group demodulator corresponds to each block or floor, and the group signal is finally separated into the alarm signals for respective subscriber terminals by low frequency filters.
In the meantime, a CATV system covers a great many subscribers ranging from thousands to hundreds of thousands of households and there must be a great number of control signals to have these many subscribers covered by signals separated for each.
However, when a number of control signals are transmitted by utilizing a bidirectional CATV system, the number of signals transmitted is determined or limited essentially by the capability of signal separation at the receiving end even when the characteristics of the transmission system are not taken into consideration. When the number of subscribers is greater than the limited number of signals, it is impossible to assign control signals to all of the subscribers, part of the subscribers failing to transmit control signals as up-signals. Thus, it becomes difficult to utilize a bidirectional CATV system effectively. Therefore, in order to enhance the usefulness and attain the effective application of a bidirectional CATV system, it is essential to assign control signals to all of the subscribers. Thus it becomes necessary to obtain as many signals as possible within a certain frequency band of up-signals.
For this reason, it is desirable for an increase in the number of control signals that the spacings between adjacent group carrier frequencies of the group modulators should be narrow as far as possible. On the other hand, from the standpoint of signal separation at the receiving end, such spacings should be wide as far as possible thus contradicting to the requirement for the group modulators. Generally, as shown in FIG. 5, the range of the frequencies assigned for a bidirectional CATA system comprises a bandwidth of from 70 MHz to 300 MHz for VHF-TV down-signals sent from the head end, a bandwidth of from 450 MHz to 800 MHz for UHF-TV down-signals sent from the head end and a bandwidth of from 10 MHz to 60 MHz for the up-signals transmitted from the subscribers end. Accordingly, in the alarm system described above in which any bandwidth in the up-signal bandwidth is used as the transmission band for control signals, or the alarm signals, and the high frequency carrier waves in said bandwidth are modulated by the low frequency control signals, ranging from 1 KHz to 10 KHz for example the bandwidth for this purpose is limited in a narrow range in the up-signal bandwidth so that if the spacings are narrowed for increasing the number of control signals in this limited bandwidth the capability of signal separation would be impaired.
In FIG. 5 reference numeral 101 denotes the up-signal frequency band of 10 to 60 MHz, reference numeral 102 the down signal VHF band of 70 to 300 MHz, and reference numeral 103 the down signal UHF band of 450 to 800 MHz. These frequency bands are examples used in a general bidirectional CATV system. Reference numeral 100 shows the control signal band of 1 to 10 KHz.
Considering now the resolution of the filter provided for the group demodulator for separating the received signals into signals corresponding to respective group carrier frequencies, the amount of alternation .omicron. of a low pass filter shown in FIG. 1 is expressed by the following equation. EQU .omicron. = cos h.sup.-1 .sqroot.1 - (.omega./.omega..sub.1).sup.2 ( 1)
where .omega..sub.1 represents the cutoff frequency, .omega. the variable frequency and .omicron. a monotonically increasing function. By simplifying equation (1) we obtain equation (2) EQU .omicron. = F (.omega./.omega..sub.1) (2)
in equation (2), where .omega. =.omega..sub.1 +.DELTA..omega., by representing the amount of attenuation at a frequency .omega. by .omicron.', we obtain EQU .omicron. = F (.omega..sub.1 +.DELTA..omega.)/.omega..sub.1 = F (1 +.DELTA..omega.)/.omega..sub.1 ( 3)
in equation (3) if .omega..sub.1 were very high, (.DELTA..omega./.omega..sub.1 ) would be small. Hence .omicron.' would also be small.
Conversely, if .omega..sub.1 is very low, (.DELTA..omega./.omega..sub.1) and .omicron.' will become large.
Accordingly, for a definite frequency spacing .DELTA..omega., lower .omega..sub.1 results in an acuter attenuation characteristic. In other words, it can be noted that signals will be separated satisfactorily when the group carrier frequencies of the group modulators are lowered so as to operate the group demodulators at relatively low frequencies.
Further considering the resolution of a double resonance circuit comprising a transformer T of which a primary side and a secondary side are resonated with a predetermined frequency f.sub.o as shown in FIG. 2, the deviation of the transmission gain A with a definite frequency spacing .DELTA.f is expressed by the following equation. EQU A = 10 log { 1 + 4 (.DELTA.f .multidot. Q.sub.L / f.sub.o).sup.2 }(4)
where Q.sub.L represents the Q factor of the transformer T. In equation (4), where Q.sub.L is constant, if f.sub.o is very high, A will be small. Conversely, if f.sub.o is very low, A will be large. Accordingly, for a definite .DELTA.f, lower f.sub.o results in a larger deviation of transmission gain of transformer T. Hence it also can be noted that signals will be separated satisfactorily as shown in FIG. 1.
However, as described above, as the bandwidth for outgoing signals assigned to the bidirectional CATV system is limited so that it is impossible to lower the transmission frequency thereof to such extent as to effectively improve the separation of the signals.